1. Field of the Invention
This invention relates to the design and preparation of semiconductor materials having skutterudite type crystal lattice structures with voids or cavities that may be selectively filled to enhance various thermoelectric properties of the semiconductor materials.
2. Description of the Related Prior Art
The basic theory and operation of thermoelectric devices has been developed for many years. Presently available thermoelectric devices used for cooling typically include an array of thermocouples which operate in accordance with the Peltier effect. Thermoelectric devices may also be used for heating, power generation and temperature sensing.
Thermoelectric devices, for cooling applications, may be described as essentially small heat pumps which follow the laws of thermodynamics in the same manner as mechanical heat pumps, refrigerators, or any other apparatus used to transfer heat energy. A principal difference is that thermoelectric devices function with solid state electrical components (thermoelectric elements or thermocouples) as compared to more traditional mechanical/fluid heating and cooling components. The efficiency of a thermoelectric device is generally limited to its associated Carnot cycle efficiency reduced by a factor which is dependent upon the thermoelectric figure of merit (ZT) of materials used in fabrication of the associated thermoelectric elements. Materials used to fabricate other components such as electrical connections, hot plates and cold plates may also affect the overall efficiency of the resulting thermoelectric device.
The thermoelectric figure of merit (ZT) is a dimensionless measure of the effectiveness of a thermoelectric device and is related to material properties by the following equation: EQU ZT=S.sup.2.sigma.T/.kappa. (1)
where S,.sigma.,.kappa., and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively. The Seebeck coefficient (S) is a measure of how readily the respective charge carriers (electrons or holes) can transfer energy as they move through a thermoelectric element which is subjected to a temperature gradient. The type of carrier (electron or hole) is a function of the dopants in the semiconductor materials selected to form each thermoelectric element.
The electrical properties (sometimes referred to as power factor, electrical characteristics, electronic properties, or electronic characteristics) associated with materials used to form thermoelectric elements may be represented by S.sup.2.sigma.. Many of the materials which are used to form thermoelectric elements may be generally described as semiconductor compounds or semiconductor materials. Examples of such materials will be discussed later in more detail.
The thermoelectric figure of merit (ZT) is also related to the strength of interactions between the carriers and vibrational modes of the crystal lattice structure (phonons) and available carrier energy states. Both the crystal lattice structure and the carrier energy states are a function of the materials selected to form each thermoelectric device. As a result, thermal conductivity (.kappa.) is a function of both an electronic component (.kappa..sub.e) primarily associated with the respective carriers and a lattice component (.kappa..sub.g) primarily associated with the respective crystal lattice structure and propagation of phonons through the respective crystal lattice structure. In the most general sense, thermal conductivity may be stated by the equation: EQU .kappa.=.kappa..sub.e +.kappa..sub.g (2)
The thermoelectric figure of merit (ZT) may also be stated by the equation: ##EQU1##
Thermoelectric materials for cooling and power generation applications such as alloys of Bi.sub.2 Te.sub.3, PbTe and BiSb were developed thirty to forty years ago. More recently, semiconductor alloys such as SiGe have been used in the fabrication of thermoelectric power generation devices. Commercially available thermoelectric materials are generally limited to use in a temperature range between 200.degree. K. and 1300.degree. K. with a maximum ZT value of approximately one. The efficiency of such thermoelectric devices remains relatively low at approximately five to eight percent (5-8%) energy conversion efficiency. For the temperature range of 100.degree. C. to 1000.degree. C., maximum ZT for many state of the art thermoelectric materials also remains limited to values of approximately 1, except for Te--Ag--Ge--Sb alloys (TAGS) which may achieve a ZT of 1.2 to 1.4 in a relatively narrow temperature range. Recently developed materials such as Si.sub.80 Ge.sub.20 alloys used in thermoelectric generators to power spacecrafts for deep space missions have an average thermoelectric figure of merit (ZT) of approximately equal to 0.5 from 300.degree. C. to 1,000.degree. C.
Many crystalline materials with low thermal conductivity do not have good electrical conductivity and many crystalline materials with good electrical conductivity often have relatively high values of thermal conductivity. For example, many binary semiconductor compounds which have skutterudite type crystal lattice structures have relatively good electrical properties. However, the value of thermal conductivity associated with the skutterudite type crystal lattice structure of such semiconductor compounds is generally relatively large which often results in a thermoelectric figure of merit (ZT) which is less than desired.
Research and development has previously been conducted on fabricating thermoelectric devices with thermoelectric elements formed from materials having skutterudite type crystal lattice structures. Examples of such developments are shown in U.S. Pat. No. 5,610,366 entitled High Performance Thermoelectric Materials and Methods of Preparation issued Mar. 11, 1997. Patent application Ser. No. 08/101,901 filed Aug. 3, 1993, entitled A Semiconductor Apparatus Utilizing Gradient Freeze and Liquid-Solid Techniques, now U.S. Pat. No. 5,769,943, and patent application Ser. No. 08/412,700 filed Mar. 29, 1995, entitled Advanced Thermoelectric Materials with Enhanced Crystal Lattice Structure and Methods of Preparation, now U.S. Pat. No. 5,747,728, show additional examples of skutterudite type crystal lattice structures which may be used to fabricate thermoelectric devices.
One reference presents X-ray analysis of synthesized samples with the major phase being partially filled phosphorous based skutterudite type crystal lattice structures. See, "Synthesis and Crystal Structure of a New Series of Ternary Phosphides in the System TR-Co-P (TR: Rare Earth)" by S. Zemni, D. Tranqui, P. Chaudouet, R. Madar, and J. P. Senateur, Journal Solid State Chemistry, 65, 1 (1986).